Multilayer laminate film

ABSTRACT

The invention provides a multilayer laminated film with alternately laminated birefringent and isotropic layers. The birefringent layers have a first monotonically increasing region of optical thickness and contain monotonically increasing region 1A of maximum optical thickness of 100 nm or less, and monotonically increasing region 1B of minimum optical thickness of more than 100 nm, and ratio 1B/1A of slope 1B of monotonically increasing region 1B to slope 1A of monotonically increasing region 1A is more than 0 and less than 0.8. The isotropic layers have a second monotonically increasing region of optical thickness and contain monotonically increasing region 2A of maximum optical thickness of 200 nm or less and monotonically increasing region 2B of minimum optical thickness of more than 200 nm, and ratio 2B/2A of slope 2B of monotonically increasing region 2B to slope 2A of monotonically increasing region 2A is more than 1.5 and 10 or less.

TECHNICAL FIELD

The present disclosure relates to a multilayer laminated film that canwidely reflect light in a visible light region.

BACKGROUND ART

A multilayer laminated film in which many layers with a low refractiveindex and many layers with a high refractive index are alternatelylaminated can be used as an optical interference film that selectivelyreflects or transmits light with a specific wavelength due to opticalinterference caused from the layered structure. Further, by graduallychanging the film thickness of each layer along the thickness direction,or by bonding together films having different reflection peaks, such amultilayer laminated film can reflect or transmit light in a widewavelength range, attain a reflectance as high as a film using metal,and be used as a metallic luster film or a reflective mirror. Further,it is known that by stretching such a multilayer laminated film in onedirection, the multilayer laminated film can also be used as areflective polarizing film that reflects only a specific polarizationcomponent, and be used, for example, as a luminance-improving member forliquid crystal displays or the like (Patent Literature (PTL) 1 to PatentLiterature (PTL) 4, etc.).

These multilayer laminated films are often required to have a higherreflectance in an arbitrary wavelength range. However, since the numberof layers that can be laminated is limited, it is very difficult toachieve a high reflectance over a broad reflection wavelength range. Inaddition, increasing only the reflectance of light in a specificwavelength range might lead to a decrease in reflectance of light inother reflection wavelength ranges, causing an optical quality problem.

CITATION LIST Patent Literature

PTL 1: JPH04-268505A

PTL 2: JPH09-506837A

PTL 3: JPH09-506984A

PTL 4: WO01/47711

SUMMARY OF INVENTION Technical Problem

On the other hand, the multilayer laminated films can be required tohave a high degree of polarization, in addition to a high degree ofreflectance. In particular, a multilayer laminated film, which isdesired to be smaller and lighter, has a limited number of layers;therefore, within a limited thickness range or limited weight range, themultilayer laminated film is required to have a high degree ofpolarization while maintaining a wide reflection wavelength range.

Coloring can be observed when these multilayer laminated films areviewed from an oblique direction, and the occurrence of such coloringalso indicates a narrow reflection wavelength range.

An object of the present disclosure is to provide a multilayer laminatedfilm having a high degree of polarization while maintaining a widereflection wavelength range.

Solution to Problem

Means for solving the problems include the following embodiments.

1. A multilayer laminated film comprising a multilayer laminate in whicha birefringent layer comprising a first resin and an isotropic layercomprising a second resin are alternately laminated,the multilayer laminated film being capable of reflecting light with awavelength of 380 to 780 nm due to optical interference caused from thelamination structure of the birefringent layer and the isotropic layer,a series of the birefringent layers having a first monotonicallyincreasing region of optical thickness, wherein the first monotonicallyincreasing region comprises a monotonically increasing region 1A inwhich the maximum optical thickness is 100 nm or less, and amonotonically increasing region 1B in which the minimum opticalthickness is more than 100 nm, and a ratio 1B/1A of a slope 1B of themonotonically increasing region 1B to a slope 1A of the monotonicallyincreasing region 1A is more than 0 and less than 0.8,a series of the isotropic layers having a second monotonicallyincreasing region of optical thickness, wherein the second monotonicallyincreasing region comprises a monotonically increasing region 2A inwhich the maximum optical thickness is 200 nm or less and amonotonically increasing region 2B in which the minimum opticalthickness is more than 200 nm, and a ratio 2B/2A of a slope 2B of themonotonically increasing region 2B to a slope 2A of the monotonicallyincreasing region 2A is more than 1.5 and 10 or less.2. The multilayer laminated film according to Item 1, wherein themonotonically increasing region 1A has an average optical thickness of65 nm or more and 85 nm or less, and the monotonically increasing region1B has an average optical thickness of 140 nm or more and 160 nm orless.3. The multilayer laminated film according to Item 1 or 2, wherein themonotonically increasing region 2A has an average optical thickness of130 nm or more and 155 nm or less, and the monotonically increasingregion 2B has an average optical thickness of 250 nm or more and 290 nmor less.4. The multilayer laminated film according to any one of Items 1 to 3,wherein the ratio 2B/2A is more than 1.5 and less than 5.5. The multilayer laminated film according to any one of Items 1 to 3,wherein the ratio 2B/2A is 5 or more and 10 or less.6. The multilayer laminated film according to any one of Items 1 to 5,having an average reflectance of light polarized parallel to areflection axis at normal incidence of 85% or more in a wavelength rangeof 380 nm to 780 nm.

Advantageous Effects of Invention

According to the present disclosure, a multilayer laminated film havinga high degree of polarization while maintaining a wide reflectionwavelength range is provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing an example of a layer thicknessprofile of a multilayer laminated film according to the presentdisclosure.

FIG. 2 is a graph showing transmission spectra in terms of thetransmission axis and the reflection axis of a multilayer laminated filmaccording to the present disclosure.

FIG. 3 is a schematic view showing an example of a layer thicknessprofile of a multilayer laminated film according to the presentdisclosure.

FIG. 4 is a graph showing transmission spectra in terms of thetransmission axis and the reflection axis of a multilayer laminated filmaccording to the present disclosure.

DESCRIPTION OF EMBODIMENTS

An embodiment, which is an example of the present disclosure, isdescribed below. The present disclosure is in no way limited to thefollowing embodiment, and can be implemented with appropriatemodifications within the scope of the present disclosure.

In the present specification, a numerical range indicated by “ . . . to. . . ” means a range including the numerical values given before andafter “to” as the lower limit and the upper limit.

Multilayer Laminated Film

The multilayer laminated film according to one embodiment of the presentdisclosure comprises a multilayer laminate in which a birefringent layercomprising a first resin and an isotropic layer comprising a secondresin are alternately laminated. The film can reflect light in a broadwavelength range of 380 to 780 nm in a visible light region due tooptical interference caused by the lamination of the birefringent layersand the isotropic layers. The film can reflect light, for example, in awavelength range of 400 to 760 nm, and preferably a wavelength range of380 to 780 nm. In the present disclosure, the phrase “can reflect” or“capable of reflecting” means that in at least one arbitrary directionon the film surface, the average reflectance at perpendicular incidenceof polarized light parallel to the direction is 50% or more. In terms ofthe average reflectance in each wavelength range, this reflection can be50% or more, preferably 60% or more, more preferably 70% or more, andstill more preferably 85% or more, and so the multilayer laminated filmmaintains a wide reflection wavelength range.

In the present disclosure, the average reflectance is a value obtainedby subtracting from 100 the average transmittance at the wavelength of380 to 780 nm, which is determined using a polarizing film measurementapparatus (VAP7070S, manufactured by JASCO Corporation).

In the present disclosure, “composed mainly of a resin” means that theresin in each layer accounts for 70 mass % or more, preferably 80 mass %or more, and more preferably 90 mass % or more, of the total mass ofeach layer.

In order to achieve such reflection properties, the multilayer laminateof alternating layers preferably has a structure in which a birefringentlayer and an isotropic layer are alternately laminated in the thicknessdirection so that the total number of the birefringent layers and theisotropic layers laminated is 30 or more, each birefringent layer beingcomposed mainly of a first resin and having a film thickness of 10 to1000 nm, and each isotropic layer being composed mainly of a secondresin and having a film thickness of 10 to 1000 nm. The resin that formsthe birefringent layers and the resin that forms the isotropic layers,which will be described in detail below, are not particularly limited aslong as they can form a layer having birefringent properties and a layerhaving isotropic properties, respectively. Both of the resins arepreferably thermoplastic resin from the viewpoint of easy production ofthe film. In the present disclosure, with respect to refractive indexesin the machine direction, the traverse direction, and the thicknessdirection, a layer having a reflective index difference of 0.1 or morebetween the maximum and the minimum is defined as being birefringent,and a layer having a reflective index difference of less than 0.1 isdefined as being isotropic.

Layer Thickness Profile

The multilayer laminated film according to one embodiment of the presentdisclosure can reflect light in a wide wavelength range by having thelaminated structure of birefringent layers and isotropic layers withvarious optical thicknesses. This is because the reflection wavelengthis due to the optical thickness of each layer that constitutes themultilayer laminated film. In general, the reflection wavelength of themultilayer laminated film is represented by the following Formula 1.

λ=2(n1×d1+n2×d2)  (Formula 1)

(In Formula 1, λ represents a reflection wavelength (nm); n1 and n2represent the refractive index of the birefringent layer and therefractive index of the isotropic layer, respectively; and d1 and d2represent the physical thickness (nm) of the birefringent layer and thephysical thickness (nm) of the isotropic layer, respectively.)

Further, an optical thickness λm (nm) is represented by the product of arefractive index nk and a physical thickness dk of each layer as in thefollowing Formula 2. For the physical thickness, one obtained from aphotograph taken with a transmission electron microscope can be used.

λM(nm)=nk×dk  (Formula 2)

In view of the above, it is possible to obtain a layer thickness profilewith which light can be widely reflected at a wavelength of from 380 to780 nm. For example, the multilayer laminated film can be designed toreflect light in a wide wavelength range by widening the thickness rangein the monotonously increasing region described below, or can bedesigned to reflect light in a specific wavelength range in themonotonously increasing region, and reflect light outside the specificwavelength range in other regions to thereby reflect light in a broadwavelength range as a whole.

In one embodiment of the present disclosure, the birefringent layer andthe isotropic layer each have a specific layer thickness profile, whichmakes it possible to obtain a multilayer laminated film having a highdegree of polarization while maintaining a wide reflection wavelengthrange.

More specifically, the layer thickness profile of the birefringentlayers in terms of optical thickness has a first monotonicallyincreasing region. The first monotonically increasing region comprises amonotonically increasing region 1A in which the maximum opticalthickness is 100 nm or less, and a monotonically increasing region 1B inwhich the minimum optical thickness is more than 100 nm, and a ratio1B/1A of the slope 1B of the monotonically increasing region 1B to theslope 1A of the monotonically increasing region 1A is more than 0 andless than 0.8. At the same time, the layer thickness profile of theisotropic layers in terms of optical thickness has a secondmonotonically increasing region. The second monotonically increasingregion comprises a monotonically increasing region 2A in which themaximum optical thickness is 200 nm or less and a monotonicallyincreasing region 2B in which the minimum optical thickness is more than200 nm, and a ratio 2B/2A of the slope 2B of the monotonicallyincreasing region 2B to the slope 2A of the monotonically increasingregion 2A is more than 1.5 and 10 or less. FIG. 1 is a schematic view ofan example of a layer thickness profile of a multilayer laminated filmaccording to the present disclosure. FIG. 2 is a graph showingtransmission spectra in terms of the transmission axis and thereflection axis of a multilayer laminated film having the layerthickness profile shown in FIG. 1. Similarly, FIG. 3 is a schematic viewof another example of a layer thickness profile of a multilayerlaminated film according to the present disclosure. FIG. 4 is a graphshowing transmission spectra in terms of the transmission axis and thereflection axis of a multilayer laminated film having the layerthickness profile shown in FIG. 3.

FIG. 1 shows a layer thickness profile in which the ratio 1B/1A of theslope 1B of the monotonically increasing region 1B to the slope 1A ofthe monotonically increasing region 1A is 0.5, and the ratio 2B/2A ofthe slope 2B of the monotonically increasing region 2B to the slope 2Aof the monotonically increasing region 2A is 3.0. FIG. 2 shows atransmission spectrum in terms of the transmission axis (dotted line)and a transmission spectrum in terms of the reflection axis (solid line)of a multilayer laminated film having the layer thickness profile shownin FIG. 1. The degree of polarization of the multilayer laminated filmcalculated from the transmission spectra of FIG. 2 is 85.0%, and theaverage transmittance of light with a wavelength of 380 to 780 nm in thedirection of the reflection axis is 9.6% (average reflectance: 90.4%).This suggests that the multilayer laminated film according to thepresent disclosure has a wide reflection wavelength range of 380 to 780nm.

Further, FIG. 3 shows a layer thickness profile in which the ratio 1B/1Aof the slope 1B of the monotonically increasing region 1B to the slope1A of the monotonically increasing region 1A is 0.5, and the ratio 2B/2Aof the slope 2B of the monotonically increasing region 2B to the slope2A of the monotonically increasing region 2A is 6.8. FIG. 4 shows atransmission spectrum in terms of the transmission axis (dotted line)and a transmission spectrum in terms of the reflection axis (solid line)of a multilayer laminated film having the layer thickness profile shownin FIG. 3. The degree of polarization of the multilayer laminated filmcalculated from the transmission spectra of FIG. 4 is 91.0%, and theaverage transmittance of light with a wavelength of 380 to 780 nm in thedirection of the reflection axis is 6.8% (average reflectance: 93.2%).This suggests that the multilayer laminated film according to thepresent disclosure has a wide reflection wavelength range of 380 to 780nm.

As shown as examples in FIGS. 1 and 3, the monotonically increasingregion 1A having the slope 1A and the monotonically increasing region 1Bhaving the slope 1B, which constitute the first monotonically increasingregion, serve as a continuous region that satisfies the ratio 1B/1A ofmore than 0 and less than 0.8, and have an optical thickness of 100 nmat the boundary. Further, the monotonically increasing region 2A havingthe slope 2A and the monotonically increasing region 2B having the slope2B, which constitute the second monotonically increasing region, serveas a continuous region that satisfies the ratio 2B/2A of more than 1.5and 10 or less, and have an optical thickness of 200 nm at the boundary.

When the ratio 1B/1A and the ratio 2B/2A of the slopes simultaneouslysatisfy the above ranges, a multilayer laminated film having a highdegree of polarization while maintaining a wide reflection wavelengthrange can be obtained. This is because when the above ratios of theslopes are simultaneously satisfied, the resulting multilayer laminatedfilm can achieve a higher reflectance while having a broader reflectionwavelength range. Conventionally, the reflection wavelength range tendsto be narrow when a high reflectance is achieved. However, in oneembodiment of the present disclosure, a high degree of polarization canbe achieved while maintaining a wide reflection wavelength range.

An increase in the number of layers theoretically can achieve a highdegree of polarization in a wide reflection wavelength range. However,an increase in the number of layers usually requires a change inequipment. In the multilayer laminated film according to thisembodiment, if the ratio 1B/1A of the slopes is adjusted to more than 0and less than 0.8, and the ratio 2B/2A of the slopes to more than 1.5and 10 or less, it is possible to produce a multilayer laminated filmthat maintains a wide reflection wavelength range and has a high degreeof polarization even with existing equipment. Additionally, it is alsopossible to produce a multilayer laminated film without changing thenumber of layers from that of existing films.

The slope of a layer thickness profile as used in the present disclosurerefers to a slope of a first approximate straight line based on thefollowing method. That is, the slope of the first approximate straightline of the layer thickness profile of the monotonically increasingregion 1A of the birefringent layer is defined as “1A,” the slope of thefirst approximate straight line of the layer thickness profile of themonotonically increasing region 1B is defined as “1B,” and 1B/1A iscalculated using the obtained values. Further, the slope of the firstapproximate straight line of the layer thickness profile of themonotonically increasing region 2A of the isotropic layer is defined as“2A,” the slope of the first approximate straight line of the layerthickness profile of the monotonically increasing region 2B is definedas “2B,” and 2B/2A is calculated using the obtained values. In oneembodiment of the present disclosure, the number of layers can beincreased by doubling or the like, as described below. In such a case,it is only necessary to look at the layer thickness profile of onepacket that can be a multilayer laminate of alternating layers. Whenlooking at the overall layer thickness profile of a multilayer laminatedfilm, for example, if there are multiple portions having similar layerthickness profiles, each portion can be regarded as a packet, andmultilayer structure portions separated by, for example, an intermediatelayer, can be regarded as separate packets.

For the birefringent layer, the optical thickness of the firstmonotonically increasing region at the boundary is set to 100 nm, andthe ratio of the slope of the monotonically increasing region 1A, whichhas a smaller optical thickness, and the slope of the monotonicallyincreasing region 1B, which has a greater optical thickness, is set tobe within a specific range. This can increase the intensity ofreflection of light with a wavelength of about 550 nm, at which light isvisually well perceived, while widening the wavelength range for thefirst monotonically increasing region, and can improve the degree ofpolarization. Setting the value at the boundary to 100 nm can furtherincrease the intensity of reflection of light with a wavelength of about550 nm. In the birefringent layer, if the optical thickness at theboundary is set to 150 nm or 200 nm, the intensity of reflection oflight in the above wavelength range will not effectively improve,resulting in a tendency of not effectively improving the degree ofpolarization.

For the isotropic layer, the optical thickness of the secondmonotonically increasing region at the boundary is set to 200 nm, andthe ratio of the slope of the monotonically increasing region 2B, whichhas a greater optical thickness, to the slope of the monotonicallyincreasing region 2A, which has a smaller optical thickness, is set tobe within a specific range. This can widen the reflection wavelengthrange, and a high degree of polarization can be easily obtained. Incontrast, if the ratio of the slopes deviates from the specific range,the reflection range becomes too wide, resulting in a tendency towards adecrease in the degree of polarization. By setting the optical thicknessat the boundary to 200 nm, and by relatively reducing the slope 2A interms of the relation between slopes 2A and 2B, it is possible to adjustthe reflection intensity to be uniform. By relatively increasing theslope 2B, it is possible to increase the reflection intensity in thedesired wavelength range by using higher-order reflection, such assecondary reflection or tertiary reflection, while widening thereflection wavelength range.

From the above viewpoints, the value of the ratio 1B/1A is more than 0and less than 0.8. For example, an embodiment in which the lower limitis 0.01, 0.02, or 0.30, an embodiment in which the upper limit is 0.79,0.70, or 0.65, and an embodiment in which any of these lower limits andupper limits are combined are preferable. More specifically, anembodiment in which the value of the ratio is 0.01 to 0.79, anembodiment in which the value is 0.02 to 0.70, an embodiment in whichthe value is 0.30 to 0.65, and the like are preferable. Further, thevalue of the ratio 2B/2A is more than 1.5 and 10 or less. For example,an embodiment in which the lower limit is 1.51, 1.60, 3, or 3.10, anembodiment in which the upper limit is 10, 9.45, 8, or 7.7, and anembodiment in which any of these lower limits and upper limits arecombined are preferable. More specifically, an embodiment in which thevalue of the ratio is 1.51 to 10, an embodiment in which the value is1.60 to 9.45, an embodiment in which the value is 3 to 8, an embodimentin which the value is 3.10 to 7.7, and the like are preferable.

In the present disclosure, the value of the ratio 2B/2A is alsopreferably, for example, more than 1.5 and less than 5. Within thisrange, for example, an embodiment in which the lower limit is 1.51,1.60, 3, or 3.10, an embodiment in which the upper limit is 4.5, 4, 3.5,or 3.2, and an embodiment in which any of these lower limits and upperlimits are combined are preferable. Further, in this disclosure, thevalue of the ratio 2B/2A is also preferably, for example, 5 or more and10 or less. Within this range, for example, an embodiment in which thelower limit is 5, 6, 7, or 7.5, an embodiment in which the upper limitis 10, 9.45, 8, or 7.7, and an embodiment in which any of these lowerlimits and upper limits are combined are preferable.

Such a layer thickness profile can be obtained, for example, byadjusting the comb teeth in the feed block.

In the first monotonically increasing region, the slope 1A of themonotonically increasing region 1A, in which the optical thickness is100 nm or less, is preferably 1.05 to 30.0, more preferably 1.10 to28.0, and still more preferably 1.20 to 26.0. Further, the slope 1B ofthe monotonically increasing region 1B, in which the optical thicknessis more than 100 nm, is preferably 0.50 to 1.20, more preferably 0.60 to1.10, and still more preferably 0.70 to 0.99. In this manner, the effectachieved by the ratio of the slopes can further improve, and the degreeof polarization can be further prevented from decreasing.

In the second monotonically increasing region, the slope 2A of themonotonically increasing region 2A, in which the optical thickness is200 nm or less, is preferably 0.80 to 2.0, more preferably 0.90 to 1.8,and still more preferably 0.95 to 1.7. Further, the slope 2B of themonotonically increasing region 2B, in which the optical thickness ismore than 200 nm, is preferably 2.0 to 11.0, more preferably 2.2 to10.5, and still more preferably 2.4 to 10.0. In this manner, the effectachieved by the ratio of the slopes can further improve, and the degreeof polarization can be further prevented from decreasing.

In the first monotonically increasing region, the layer at the end onthe side in which the optical thickness is smaller in the monotonicallyincreasing region 1A preferably has an optical thickness (nm) of 40 to60, more preferably 43 to 57, and still more preferably 46 to 54.Further, the layer at the end on the side in which the optical thicknessis greater in the monotonically increasing region 1B preferably has anoptical thickness of 180 to 220, more preferably 185 to 215, and stillmore preferably 190 to 210. In this manner, the effect achieved by theratio of the slopes can further improve, and the degree of polarizationcan be further prevented from decreasing. Additionally, the reflectionwavelength range can be widened.

In the second monotonically increasing region, the layer at the end onthe side in which the optical thickness is smaller in the monotonicallyincreasing region 2A preferably has an optical thickness (nm) of 70 to90, more preferably 74 to 86, and still more preferably 78 to 82.Further, the layer at the end on the side in which the optical thicknessis greater in the monotonically increasing region 2B preferably has anoptical thickness of 295 to 385, more preferably 310 to 370, and stillmore preferably 325 to 355. In this manner, the effect achieved by theratio of the slopes can further improve, and the degree of polarizationcan be further prevented from decreasing. Additionally, the reflectionwavelength range can be widened.

In the first monotonically increasing region of the birefringent layer,the monotonically increasing region 1A preferably has an average opticalthickness (also referred to below as the “average optical thickness”) of65 nm to 85 nm, and the monotonically increasing region 1B preferablyhas an average optical thickness of 140 nm to 160 nm. Thereby, theeffect described above achieved by the layer thickness profile of thebirefringent layer is more easily obtained, and the degree ofpolarization is more effectively prevented from decreasing.

In the second monotonically increasing region of the isotropic layer,the monotonically increasing region 2A preferably has an average opticalthickness (also referred to below as the “average optical thickness”) of130 nm to 155 nm, and the monotonically increasing region 2B preferablyhas an average optical thickness of 250 nm to 290 nm. Thereby, theeffect described above achieved by the layer thickness profile of theisotropic layer is more easily obtained, and the degree of polarizationis more effectively prevented from decreasing.

Furthermore, when these values are all within the above ranges at thesame time, a multilayer laminated film exhibiting a higher degree ofpolarization can be obtained.

To more easily achieve the above effects, the average opticalthicknesses of the monotonically increasing region 1A and themonotonically increasing region 1B of the birefringent layer arepreferably 67 nm to 83 nm and 143 nm to 157 nm, respectively. Thethicknesses are more preferably 69 nm to 81 nm and 146 nm to 154 nm,respectively.

To more easily achieve the above effects, the average opticalthicknesses of the monotonically increasing region 2A and themonotonically increasing region 2B of the isotropic layer are preferably133 nm to 152 nm and 255 nm to 285 nm, respectively. The thicknesses aremore preferably 136 nm to 149 nm and 260 nm to 280 nm, respectively.

Monotonically Increasing Region

In the present disclosure, “monotonically increasing” preferably meansthat in the entire multilayer laminate of alternating layers of themultilayer laminated film, a thicker-side layer is thicker than athinner-side layer; however, this is not limitative. It is sufficient aslong as there is a tendency for the thickness to increase from thethinner side to the thicker side as seen in the entire view. Morespecifically, when the layers are numbered from the thinner side to thethicker side in terms of optical thickness, and the film thickness ofeach layer is plotted on the ordinate with the layer number of eachnumbered layer being plotted on the abscissa, the number of layerswithin the range showing a tendency of increasing film thickness isequally divided into five. If the average values of the film thicknessesin each equally divided area all increase in the direction in which thefilm thickness increases, the tendency is regarded as a monotonicincrease; if this is not the case, the tendency is not regarded as amonotonic increase. Note that the birefringent layers and the isotropiclayers can be viewed individually, and the monotonic increase of thebirefringent layers and the monotonic increase of the isotropic layersmay have different slopes. Moreover, the monotonic increase describedabove may be in an embodiment in which the thickness monotonicallyincreases entirely from one outermost layer to the other outermost layerin the multilayer laminate of alternating layers. In some embodiments,the monotonously increasing thickness region may account for 80% ormore, preferably 90% or more, and more preferably 95% or more, of themultilayer laminate of alternating layers in terms of the number oflayers; and the thickness in the remaining portion may be constant, ordecrease. For example, Example 1 according to the present disclosure isan embodiment in which the thickness monotonically increases in the 100%portion of the multilayer laminated structure. In some embodiments, themultilayer laminated film may include a region where the thickness doesnot monotonically increase at the smaller layer number side and/or thelarger layer number side of the thickness profile described above.

In one embodiment of present disclosure, a region in which the ratio1B/1A above is more than 0 and less than 0.8 in the monotonicallyincreasing region of the birefringent layer is referred to as a “firstmonotonically increasing region,” while a region in which the ratio2B/2A above is more than 1.5 and 10 or less in the monotonicallyincreasing region of the isotropic layer is referred to as a “secondmonotonically increasing region.”

The birefringent layer and isotropic layer are alternately laminated toform a multilayer laminate. Thus, it is sufficient as long as themonotonically increasing regions of the birefringent layer and theisotropic layer have a range capable of reflecting light with awavelength of 380 to 780 nm due to optical interference caused from themultilayer laminate structure. Further, the monotonically increasingregions of the birefringent layer and the isotropic layer may be largerthan the range that is capable of reflecting light with a wavelength of380 to 780 nm when a multilayer laminate of alternating layers isformed.

Structure of Multilayer Laminated Film Birefringent Layer

The birefringent layer of the multilayer laminated film according to oneembodiment of the present disclosure has birefringent properties. Thatis, the resin that forms the birefringent layer (also referred to as the“first resin” in the present disclosure) is capable of formingbirefringent layers. Accordingly, the resin that forms the birefringentlayer is preferably an oriented crystalline resin, and the orientedcrystalline resin is especially preferably a polyester. The polyesterpreferably contains ethylene terephthalate units and/or ethylenenaphthalate units, more preferably ethylene naphthalate units, in anamount in the range of 80 mol % or more to 100 mol % or less, based onthe repeating units constituting the polyester; this is because a layerhaving a higher refractive index can thereby be readily formed, whichmakes it easy to increase the difference in refractive index between thebirefringent layer and the isotropic layer. Here, in the case of thecombined use of resins, the content is the total content.

Polyester for Birefringent Layer

A preferred polyester for birefringent layers contains anaphthalenedicarboxylic acid component as a dicarboxylic acid component,and the content of the naphthalenedicarboxylic acid component ispreferably 80 mol % or more and 100 mol % or less, based on thedicarboxylic acid component of the polyester. Examples of thenaphthalenedicarboxylic acid component include a2,6-naphthalenedicarboxylic acid component, a2,7-naphthalenedicarboxylic acid component, components derived from acombination of these components, and derivative components thereof.Particularly preferred examples include a 2,6-naphthalenedicarboxylicacid component and derivative components thereof. The content of thenaphthalenedicarboxylic acid component is preferably 85 mol % or more,more preferably 90 mol % or more; and is preferably less than 100 mol %,more preferably 98 mol % or less, and even more preferably 95 mol % orless.

The polyester for birefringent layers may further contain a terephthalicacid component, an isophthalic acid component, or the like, especiallypreferably a terephthalic acid component, as a dicarboxylic acidcomponent of the polyester for birefringent layers, in addition to thenaphthalenedicarboxylic acid component as long as the object of thepresent disclosure is not impaired. The content of the additionaldicarboxylic acid component is preferably in the range of more than 0mol % and 20 mol % or less, more preferably 2 mol % or more, and evenmore preferably 5 mol % or more; and is more preferably 15 mol % orless, and even more preferably 10 mol % or less.

When the multilayer laminated film is used as a luminance-improvingmember or a reflective polarizer for use in a liquid crystal display orthe like, it is preferred that the birefringent layers have relativelyhigher refractive index properties than the isotropic layers, that theisotropic layers have relatively lower refractive index properties thanthe birefringent layers, and that the film be stretched in a uniaxialdirection. In this case, in the present disclosure, the uniaxiallystretching direction may be referred to as the “X direction,” thedirection orthogonal to the X direction on the film plane may bereferred to as the “Y direction” (also referred to as the“non-stretching direction”), and the direction perpendicular to the filmplane may be referred to as the “Z direction” (also referred to as the“thickness direction”).

When the birefringent layer comprises a polyester containing anaphthalenedicarboxylic acid component as the main component asdescribed above, the birefringent layer can show a high refractive indexin the X direction, and also simultaneously achieve high birefringencecharacteristics with high uniaxial orientation; this can increase therefractive index difference in the X direction between the birefringentlayer and the isotropic layer, thus contributing to a high degree ofpolarization. In contrast, if the content of the naphthalenedicarboxylicacid component is less than the lower limit, amorphous properties tendto increase; and the difference between a refractive index in the Xdirection, nX, and a refractive index in the Y direction, nY, tends todecrease. Therefore, the multilayer laminated film is less likely toobtain satisfactory reflection performance of the P-polarized lightcomponent (in the present disclosure), which is defined as a polarizedlight component being parallel to the incidence plane including theuniaxially stretching direction (X direction), with the film surfacebeing used as a reflection surface. In the multilayer laminated film,the S-polarized light component (in the present disclosure) is definedas a polarized light component being perpendicular to the incidenceplane that includes the uniaxially stretching direction (X direction),with the film surface being used as a reflection surface.

As the diol component of a preferred polyester for birefringent layers,an ethylene glycol component is used. The content of the ethylene glycolcomponent is preferably 80 mol % or more and 100 mol % or less, morepreferably 85 mol % or more and 100 mol % or less; and even morepreferably 90 mol % or more and 100 mol % or less, and particularlypreferably 90 mol % or more and 98 mol % or less, based on the diolcomponent of the polyester. If the amount of the diol component is lessthan the lower limit, the uniaxial orientation described above may beimpaired.

The polyester for birefringent layers may further contain a trimethyleneglycol component, a tetramethylene glycol component, acyclohexanedimethanol component, a diethylene glycol component, or thelike as a diol component of the polyester for birefringent layers, inaddition to the ethylene glycol component, as long as the object of thepresent disclosure is not impaired.

Properties of Polyester for Birefringent Layer

The melting point of the polyester for birefringent layers is preferablyin the range of 220 to 290° C., more preferably 230 to 280° C., and evenmore preferably 240 to 270° C. The melting point can be determined bymeasurement using a differential scanning calorimeter (DSC). When themelting point of the polyester is more than the upper limit, fluidity islikely to be poor when molding through melt extrusion is performed, thuscausing extrusion or the like to be non-uniform. On the other hand, ifthe melting point is less than the lower limit, excellent filmformability is attained, but the mechanical properties etc. of thepolyester are likely to worsen; additionally, it tends to be difficultfor the film to exhibit the refractive index properties required whenused as a luminance-improving member or a reflective polarizer for aliquid crystal display.

The glass transition temperature (sometimes referred to below as “Tg”)of the polyester used for birefringent layers is preferably in the rangeof 80 to 120° C., more preferably 82 to 118° C., even more preferably 85to 118° C., and particularly preferably 100 to 115° C. When Tg is inthis range, the resulting film has excellent heat resistance anddimensional stability, and readily exhibits the refractive indexproperties required when used as a luminance-improving member or areflective polarizer for a liquid crystal display. The melting point andthe glass transition temperature can be adjusted by controlling, forexample, the type and amount of copolymer component, and diethyleneglycol, which is a by-product.

The polyester used for birefringent layers preferably has an intrinsicviscosity of 0.50 to 0.75 dl/g, more preferably 0.55 to 0.72 dl/g, andeven more preferably 0.56 to 0.71 dl/g, as measured at 35° C. using ano-chlorophenol solution. By having such an intrinsic viscosity, thebirefringent layer tends to readily have appropriately orientedcrystallinity, and a difference in the refractive index between thebirefringent layer and the isotropic layer tends to easily increase.

Isotropic Layer

The isotropic layer of the multilayer laminated film according to oneembodiment of the present disclosure is a layer having isotropicproperties. That is, the resin of the isotropic layer (also referred toas the “second resin” in the present disclosure) is capable of formingisotropic layers. Thus, the resin that forms the isotropic layer ispreferably an amorphous resin. In particular, an amorphous polyester ispreferred. The term “amorphous” as used herein does not exclude a resinhaving slight crystalline properties, but includes any resin that canmake the layer isotropic to an extent that the multilayer laminated filmaccording to the present invention can have the intended function.

Copolyester for Isotropic layer

The resin that forms the isotropic layers is preferably a copolyester.It is particularly preferable to use a copolyester containing anaphthalenedicarboxylic acid component, an ethylene glycol component,and a trimethylene glycol component as copolymer components. Examples ofthe naphthalenedicarboxylic acid component include a2,6-naphthalenedicarboxylic acid component, a2,7-naphthalenedicarboxylic acid component, components derived from acombination of these components, and derivative components thereof.Particularly preferred examples include a 2,6-naphthalenedicarboxylicacid component and derivative components thereof. The copolymercomponent as referred to herein means any of the components thatconstitute the polyester. The copolymer component is not limited to acopolymer component as a minor component (which is used in an amount forcopolymerization of less than 50 mol %, based on the total amount of theacid component or the total amount of the diol component), and alsoincludes a main component (which is a component used in an amount forcopolymerization of 50 mol % or more, based on the total amount of theacid component or the total amount of the diol component).

In one embodiment of the present disclosure, a polyester having anethylene naphthalate unit as a main component is preferably used as aresin for isotropic layers, as described above. This is preferablebecause the use of a copolyester containing a naphthalenedicarboxylicacid component as the resin for isotropic layers increases thecompatibility with birefringent layers, and tends to improve interlayeradhesion to the birefringent layers, and so delamination is less likelyto occur.

The copolyester for isotropic layers preferably contains at least twocomponents, i.e., an ethylene glycol component and a trimethylene glycolcomponent, as diol components. Of these, the ethylene glycol componentis preferably used as the main diol component from the viewpoint of filmformability etc.

The copolyester for isotropic layers in one embodiment of the presentdisclosure preferably further contains a trimethylene glycol componentas a diol component. The presence of a trimethylene glycol component inthe copolyester compensates for the elasticity of the layer structure toenhance the effect of suppressing delamination.

The naphthalenedicarboxylic acid component, preferably a2,6-naphthalenedicarboxylic acid component, preferably accounts for 30mol % or more and 100 mol % or less, more preferably 30 mol % or moreand 80 mol % or less, and even more preferably 40 mol % or more and 70mol % or less, of the entire carboxylic acid component of thecopolyester for isotropic layers. Using this component in the aboverange can further increase the adhesion to the birefringent layer. Ifthe content of the naphthalenedicarboxylic acid component is less thanthe lower limit, lower adhesion may result in view of compatibility. Theupper limit of the content of the naphthalenedicarboxylic acid componentis not particularly limited; however, if the amount is too large, ittends to be difficult to increase a difference in refractive indexbetween the birefringent layer and the isotropic layer. In order toadjust the relationship between the refractive index of the birefringentlayer and the refractive index of the isotropic layer, otherdicarboxylic acid components may also be copolymerized.

The amount of the ethylene glycol component is preferably 50 mol % ormore and 95 mol % or less, more preferably 50 mol % or more and 90 mol %or less, even more preferably 50 mol % or more and 85 mol % or less, andparticularly preferably 50 mol % or more and 80 mol % or less of theentire diol component of the copolyester for isotropic layers. By usingthis component in the above range, a difference in refractive indexbetween the birefringent layer and the isotropic layer tends to easilyincrease.

The amount of the trimethylene glycol component is preferably 3 mol % ormore and 50 mol % or less, more preferably 5 mol % or more and 40 mol %or less, even more preferably 10 mol % or more and 40 mol % or less, andparticularly preferably 10 mol % or more and 30 mol % or less, of theentire diol component of the copolyester for isotropic layers. Usingthis component in the above range can further increase the interlayeradhesion to the birefringent layer; furthermore, a difference inrefractive index between the birefringent layer and the isotropic layertends to easily increase. If the content of the trimethylene glycolcomponent is less than the lower limit, ensuring the interlayer adhesiontends to be difficult. If the content of the trimethylene glycolcomponent is more than the upper limit, it is difficult to obtain aresin having the desired refractive index and glass transitiontemperature.

The isotropic layer in one embodiment of the present disclosure maycontain a thermoplastic resin other than the copolyester as anadditional polymer component in an amount in the range of 10 mass % orless, based on the mass of the isotropic layer, as long as the object ofthe present disclosure is not impaired.

Properties of Polyester for Isotropic Layer

In one embodiment of the present disclosure, the copolyester forisotropic layers described above preferably has a glass transitiontemperature of 85° C. or more, more preferably 90° C. or more and 150°C. or less, even more preferably 90° C. or more and 120° C. or less, andparticularly preferably 93° C. or more and 110° C. or less. Thisprovides more excellent heat resistance. In addition, a difference inrefractive index between the birefringent layer and the isotropic layertends to easily increase. If the glass transition temperature of thecopolyester for isotropic layers is less than the lower limit,sufficient heat resistance may not be obtained. For example, whensubjected to a process including a step of heat treatment at about 90°C. or the like, the isotropic layer is likely to suffer crystallizationor embrittlement, thereby increasing haze; accordingly, the resultingfilm may exhibit a lower degree of polarization when used as aluminance-improving member or a reflective polarizer. On the other hand,when the glass transition temperature of the copolyester for isotropiclayers is too high, stretching is also likely to impart birefringence tothe polyester for isotropic layers due to stretching; accordingly, thedifference in the refractive index in the stretching direction betweenthe birefringent layer and the isotropic layer is reduced, thus causingthe reflection performance to be poor.

Among the copolyesters mentioned above, amorphous copolyesters arepreferred from the viewpoint of extremely excellent suppression of hazeincrease caused by crystallization in a heat treatment at a temperatureof 90° C. for 1000 hours. The term “amorphous” as used herein means thatwhen the temperature is increased at a temperature increase rate of 20°C./minute in measurement using a DSC, the heat of crystal fusion is lessthan 0.1 mJ/mg.

Specific examples of the copolyester for isotropic layers include (1) acopolyester containing a 2,6-naphthalenedicarboxylic acid component as adicarboxylic acid component, and an ethylene glycol component and atrimethylene glycol component as diol components; and (2) a copolyestercontaining a 2,6-naphthalenedicarboxylic acid component and aterephthalic acid component as dicarboxylic acid components, and anethylene glycol component and a trimethylene glycol component as diolcomponents.

The copolyester for isotropic layers preferably has an intrinsicviscosity of 0.50 to 0.70 dl/g, more preferably 0.55 to 0.65 dl/g, asmeasured using an o-chlorophenol solution at 35° C. When the copolyesterused for isotropic layers has a trimethylene glycol component as acopolymer component, the film-forming properties may be poor. Thefilm-forming properties can be enhanced by using a copolyester having anintrinsic viscosity within the above range. The intrinsic viscosity ofthe copolyester used as the isotropic layer is preferably higher fromthe viewpoint of film-forming properties; however, when the intrinsicviscosity is higher than the upper limit, the difference in meltviscosity between the polyester for birefringent layers and thepolyester for isotropic layers increases, which may cause the thicknessof the layers to be non-uniform.

Other Layers Outermost Layer

The multilayer laminated film according to one embodiment of the presentdisclosure can comprise an outermost layer on one or both of itssurfaces. The outermost layer is composed mainly of a resin. Here, thephrase “composed mainly of a resin” means that a resin accounts for 70mass % or more, preferably 80 mass % or more, and more preferably 90mass % or more, of the total mass of the layer. The outermost layer ispreferably an isotropic layer. The outermost layer may be composed ofthe same resin as that for isotropic layers from the viewpoint of easyproduction, and can be formed of the copolyester for isotropic layers;such an embodiment is preferred.

Intermediate Layer

The multilayer laminated film according to one embodiment of the presentdisclosure may comprise one or more intermediate layers.

In the present disclosure, the intermediate layer may also be referredto as, for example, the “inner thick layer” and means a thick layerpresent inside of the alternately laminated structure of thebirefringent layer and the isotropic layer. The term “thick” as usedherein means that the film is optically thick. In the presentdisclosure, a method is preferably used in which a thick layer (whichmay be referred to as “thickness adjustment layer” or “buffer layer”) isformed on both sides of the alternately laminated structure in theinitial stage of the production of the multilayer laminated film, andthe number of layers laminated is then increased by doubling. In thiscase, two thick layers are laminated to form an intermediate layer; athick layer formed inside is referred to as an “intermediate layer,” anda thick layer formed outside is referred to as an “outermost layer.”

The intermediate layer preferably has a layer thickness of, for example,5 μm or more and 100 μm or less, and more preferably 50 μm or less. Whensuch an intermediate layer is provided in part of the alternatelylaminated structure of the birefringent layer and the isotropic layer,the thickness of the layers constituting the birefringent layers and theisotropic layers can be easily adjusted to be made uniform withoutaffecting the polarization function. The intermediate layer may have thesame composition as the composition of the birefringent layers or thecomposition of the isotropic layers, or may have a composition thatpartially includes the composition of the birefringent layers or thecomposition of the isotropic layers. The intermediate layer is thick,and thus does not contribute to the reflection properties. On the otherhand, the intermediate layer may affect the light transmissionproperties; therefore, when the layer contains particles, the particlediameter and the particle concentration can be selected in considerationof light transmittance.

If the thickness of the intermediate layer is less than the lower limit,the layer structure of the multilayer structure may be disordered, andthe reflection performance may be reduced. On the other hand, if thethickness of the intermediate layer is more than the upper limit, theentire multilayer laminated film may be too thick, which makes itdifficult to save space when the film is used as a reflective polarizeror a luminance-improving member for a thin liquid crystal display. Whenthe multilayer laminated film contains a plurality of intermediatelayers, the thickness of each intermediate layer is preferably not lessthan the lower limit of the range of the thickness described above, andthe total thickness of the intermediate layers is preferably not greaterthan the upper limit of the range of the thickness described above.

The polymer used for the intermediate layer may be a resin differentfrom the resin for birefringent layers or the resin for isotropiclayers, as long as the polymer can be incorporated into the multilayerstructure by using the method for producing the multilayer laminatedfilm according to the present disclosure. From the viewpoint of theinterlayer adhesion, the resin preferably has the same composition asthat of either the birefringent layer or the isotropic layer, or acomposition partially including the composition of either thebirefringent layer or the isotropic layer.

The method for forming the intermediate layer is not particularlylimited. For example, a thick layer is provided on both sides of thealternately laminated structure before doubling, which is divided intotwo in the direction perpendicular to the alternately laminateddirection by using a branch block called a layer doubling block, and thedivided layers are laminated again in the alternately laminateddirection, so that one intermediate layer can be provided. A pluralityof intermediate layers can also be provided by dividing the alternatelylaminated structure into three or four by a similar technique.

Coating Layer

The multilayer laminated film according to one embodiment of the presentdisclosure can have a coating layer on at least one surface of the film.

Examples of such coating layers include a high-slipperiness layer forimparting slipperiness; a primer layer for imparting adhesion to a prismlayer, a diffusion layer, etc.; and the like. The coating layer containsa binder component and may contain, for example, particles, in order toimpart slipperiness. To impart easy adhesion, for example, a bindercomponent chemically close to the component of the layer to be adheredmay be used. The coating liquid for forming the coating layer ispreferably a water-based coating liquid using water as a solvent, fromthe environmental point of view, and particularly in such a case orother cases, for the purpose of improving wettability of the coatingliquid onto the multilayer laminated film, the coating liquid cancontain a surfactant. A functional agent may also be added; for example,a crosslinking agent may be added to improve the strength of the coatinglayer.

Method for Producing Multilayer Laminated Film

The method for producing the multilayer laminated film according to oneembodiment of the present disclosure is described below in detail. Theproduction method described below is an example, and the presentdisclosure is not limited to this. Further, different embodiments of thefilm can be obtained with reference to the following method.

The multilayer laminated film according to one embodiment of the presentdisclosure can be obtained by the following method. After a polymer forforming birefringent layers and a polymer for forming isotropic layersare alternately laminated in a molten state using a multilayer feedblock device to form an alternately laminated structure comprising, forexample, 30 layers or more in total, a buffer layer is formed on bothsides of the laminated structure. The alternately laminated structurehaving the buffer layers is then divided into, for example, two to fourby using an apparatus called “layer doubling,” and the divided layersare laminated again with the alternately laminated structure having thebuffer layers as one block; therefore, the number of laminated blocks(the number of doublings) becomes two- to four-fold, thereby increasingthe number of laminated layers. According to this method, it is possibleto obtain a multilayer laminated film comprising an intermediate layer,which is formed of a laminate of two buffer layers, inside of themultilayer structure, and an outermost layer, which is composed of onebuffer layer, on both sides of the multilayer structure.

This multilayer structure is formed by laminating the layers in such amanner that the thickness of the birefringent layers and the thicknessof the isotropic layers each have a desired inclination in thicknessprofile. This can be achieved, for example, by changing the width orlength of slits in a multilayer feed block device. For example, thebirefringent layers and isotropic layers have a different slope changerate in at least two optical thickness regions. Accordingly, the widthor length of the slits in the multilayer feed block may be adjusted sothat the at least two optical thickness regions each have at least oneor more inflection points.

After a desired number of layers are laminated by the method describedabove, the layers are extruded from a die and cooled on a casting drumto obtain a multilayer unstretched film. The multilayer unstretched filmis preferably stretched in at least one axial direction (this one axialdirection being along the film surface) selected from the axialdirection of the film-forming machine, and the direction orthogonalthereto on the film surface (which may be referred to as “traversedirection,” “width direction”, or “TD”). The stretching temperature ispreferably in the range of a glass transition temperature (Tg) of thepolymer for birefringent layers to (Tg+20°) C. The orientationproperties of the film can be more precisely controlled by stretchingthe film at a temperature lower than a conventional stretchingtemperature.

The stretch ratio is preferably from 2.0- to 7.0-fold, and morepreferably from 4.5- to 6.5-fold. Within this range, the greater thestretch ratio is, the smaller is the variation in the refractive indexin the surface direction of the individual layers of the birefringentlayers and isotropic layers due to the thinning by the stretching, lightinterference of the multilayer laminated film becomes uniform in thesurface direction, and the difference in refractive index between thebirefringent layer and the isotropic layer in the stretching directionpreferably increases. The stretching method used for this stretching canbe a known stretching method, such as heat stretching using a rodheater, roll heat stretching, or tenter stretching. Tenter stretching ispreferable from the viewpoint of, for example, reduction in scratchesdue to contact with a roller, and stretching speed.

When the film is also subjected to a stretching process in the directionorthogonal to the stretching direction on the film surface (Y-direction)to perform biaxial stretching, the stretch ratio is preferably as low asabout 1.01- to 1.20-fold, in order to impart reflective polarizationproperties to the film; however, the desired stretch ratio variesdepending on the purpose of use. If the stretching ratio in theY-direction is further increased, the polarization performance maydeteriorate.

Further, the orientation properties of the obtained multilayer laminatedfilm can be more precisely controlled by toe-out (re-stretching) in thestretching direction in the range of 5 to 15% after stretching, whileperforming heat setting at a temperature of Tg to (Tg+30°)° C.

In one embodiment of the present disclosure, when the coated layerdescribed above is provided, the application of the coating liquid tothe multilayer laminated film can be performed at any stage, and ispreferably performed during the film production process. The coatingliquid is preferably applied to the film before stretching.

The multilayer laminated film according to one embodiment of the presentdisclosure is thus obtained.

When the multilayer laminated film is to be used for a metallic lusterfilm or a reflective mirror, the film is preferably a biaxiallystretched film. In this case, either a sequential biaxial stretchingmethod or a simultaneous biaxial stretching method can be used. Thestretch ratio may be adjusted so that the refractive index and the filmthickness of the birefringent layers and those of isotropic layersprovide the desired reflection properties. For example, in considerationof general refractive indexes of the resins forming these layers, thestretch ratio may be about 2.5- to 6.5-fold in both the machinedirection and the traverse direction.

Use

Below, preferable use of the multilayer laminated film according to thepresent disclosure will be described. It is particularly preferable thatthe multilayer laminated film according to the present disclosure beused as a luminance-improving member or reflective polarizer.

Use as a Luminance-Improving Member

The multilayer laminated film according to the present disclosure inwhich the above embodiments in terms of the polymer composition, layerstructure, and orientation are adopted exhibits properties ofselectively reflecting one polarized light component and selectivelytransmitting the other polarized light component perpendicular to theabove polarized light component. More specifically, this embodiment isdirected to a uniaxially stretched multilayer laminated film. With suchproperties, the multilayer laminated film can be used as aluminance-improving member for liquid crystal displays or the like. Whenthe multilayer laminated film is used as a luminance-improving member,one polarized light component is transmitted, and the other polarizedlight component that is not transmitted, is reflected to the lightsource side without being absorbed, and so the light can be reused,exerting an excellent luminance-improving effect.

Further, a curable resin layer such as a prism layer or a diffusionlayer may be laminated on at least one surface of the multilayerlaminated film according to the present disclosure. The curable resinlayer as used herein refers to a thermosetting resin layer or anelectron beam-curable resin layer. In this embodiment, a prism layer ordiffusion layer can be laminated via a coating layer having a primerfunction or the like, which is preferable.

By bonding a part such as a prism layer to the multilayer laminated filmaccording to the present disclosure, or by forming a prism layer or thelike on a surface of the multilayer laminated film according to thepresent disclosure, to form a single unit, the number of parts to beassembled can be reduced, and the thickness of liquid crystal displayscan be reduced. Further, by bonding these parts to the multilayerlaminated film according to the present disclosure, delamination due toan external force applied during processing or the like can besuppressed, and so a more reliable luminance-improving member can beprovided. The multilayer laminated film according to the presentdisclosure may be used as a luminance-improving member, for example, inan embodiment of a liquid crystal display device comprising aluminance-improving member disposed between a light source for liquidcrystal display and a liquid crystal panel composed of a polarizer,liquid crystal cell, and polarizer. To further provide a prism layer orprism, it is preferable to dispose the prism layer or prism on theluminance-improving member on the liquid crystal panel side.

Use as a Reflective Polarizer

The multilayer laminated film according to the present disclosure, aloneor in combination with an absorptive polarizer, can be used as apolarizer for liquid crystal displays or the like. In particular, amultilayer laminated film having improved reflective polarizingperformance and a degree of polarization (P) (described later) as highas 85% or more, preferably 90% or more, and more preferably 99.5% ormore can be used alone as a polarizer for liquid crystal display that isused adjacent to a liquid crystal cell without using an absorptivepolarizer in combination.

More specifically, examples of the use of the laminated multilayer filmaccording to the present disclosure include a liquid crystal display inwhich a first polarizer comprising the laminated multilayer filmaccording to the present disclosure, a liquid crystal cell, and a secondpolarizer are laminated in this order.

EXAMPLES

Embodiments of the present disclosure are described below with referenceto Examples; however, the present disclosure is not limited to theExamples shown below. The physical properties and characteristics in theExamples were measured or evaluated by the following methods.

(1) Thickness of Each Layer

A multilayer laminated film was cut out to a size of 2 mm in thelongitudinal direction of the film and 2 cm in the width direction,fixed to an embedding capsule, and then embedded in an epoxy resin(Epomount, manufactured by Refine Tec Ltd.). The embedded sample was cutperpendicularly to the width direction with a microtome (Ultracut-UCT,manufactured by Leica) to obtain a thin section with a thickness of 50nm. The thin section of the film was observed and photographed at anaccelerating voltage of 100 kV using a transmission electron microscope(Hitachi S-4300). The thickness (physical thickness) of each layer wasmeasured from the photograph.

With respect to the layers having a thickness of more than 1 μm, a layerpresent inside the multilayer structure was regarded as an intermediatelayer, and a layer present on the outermost surface layer of themultilayer structure was regarded as an outermost layer. The thicknessof each of the layers was measured.

The optical thickness of the birefringent layers and that of isotropiclayers were each calculated by substituting the physical thickness valueof each layer obtained above and the refractive index (nX) value of eachlayer calculated according to the following (2) into the above Formula2. For the birefringent layers, the average optical thicknesses of themonotonically increasing region were calculated with respect to a regionin which the optical thickness was 100 nm or less from an end on theside in which the optical thickness was smaller and a region in whichthe optical thickness was more than 100 nm from an end on the side inwhich the optical thickness was greater. Similarly, for the isotropiclayer, the average optical thicknesses of the monotonically increasingregion were calculated with respect to a region in which the opticalthickness was 200 nm or less from an end on the side in which theoptical thickness was smaller and a region in which the opticalthickness was more than 200 nm from an end on the side in which theoptical thickness was greater.

Whether each layer is a birefringent layer or an isotropic layer can bedetermined based on the refractive index. When it is difficult todetermine, it can be determined based on NMR analysis or the electronicstate by TEM analysis.

(2) Refractive Index after Stretching in Each Direction

The refractive indexes of the birefringent layer and isotropic layer ofthe multilayer laminated film were calculated as follows. Specifically,a two-layer laminated film with a layer thickness ratio of 1:1 wasproduced under the same production conditions as those of the obtainedmultilayer laminated film. The refractive indexes of the birefringentlayer and the isotropic layer of the two-layer laminated film weremeasured as the refractive indexes of the birefringent layer and theisotropic layer of the multilayer laminated film, respectively.

For example, in this embodiment, a film having a total thickness of 75μm was produced under the same conditions as in Example 1 describedlater, except that the film was a two-layer laminated film having athickness ratio of birefringent layer:isotropic layer of 1:1. For thebirefringent layer and isotropic layer, refractive indexes in thestretching direction (X-direction), the direction orthogonal thereto(Y-direction), and the thickness direction (Z-direction) (individuallyreferred to as “nX”, “nY”, and “nZ”) were measured at a wavelength of633 nm using a Metricon prism coupler, and the obtained values were usedas the refractive indexes of the birefringent layer and the isotropiclayer after stretching.

(3) Determination of Monotonic Increase

In an arbitrary region of layer thickness profiles, which wereindividually prepared by imputing the optical thickness of thebirefringent layers or the optical thickness of the isotropic layers onthe ordinate and the layer number of each layer on the abscissa, thenumber of the layers of the birefringent layers or the isotropic layerswithin the range showing an increasing tendency in film thickness wasequally divided into five. If the average values of the film thicknessesin each equally divided area all increased in the direction in which thefilm thickness increased, the tendency was regarded as a monotonicincrease; if this was not the case, the tendency was not regarded as amonotonic increase.

In the monotonically increasing region of the birefringent layers, thelayer at the end on the side in which the optical thickness was smallerand the layer at the end on the side in which the optical thickness wasgreater were determined. Further, the slope of the first approximatestraight line of the layer thickness profile of the region in which theoptical thickness was 100 nm or less from the end on the side in whichthe optical thickness was smaller was defined as “1A,” and the slope ofthe first approximate straight line of the layer thickness profile ofthe region in which the optical thickness was more than 100 nm to theend on the side in which the optical thickness was greater was definedas “1B.” In the monotonically increasing region of the isotropic layers,the layer at the end on the side in which the optical thickness wassmaller and the layer at the end on the side in which the opticalthickness was greater were determined. Further, the slope of the firstapproximate straight line of the layer thickness profile of the regionin which the optical thickness was 200 nm or less from the end on theside in which the optical thickness was smaller was defined as “2A,” andthe slope of the first approximate straight line of the layer thicknessprofile of the region in which the optical thickness was more than 200nm to the end on the side in which the optical thickness was greater wasdefined as “2B.” From these obtained values, 1B/1A and 2B/2A werecalculated.

(4) Average Reflectance

The reflection spectrum of the obtained multilayer laminated film wasmeasured using a polarized-film measuring device (VAP7070S; manufacturedby JASCO Corporation). For the measurement, a spot diameter adjustingmask Φ 1.4 and an angle-adjustable stage were used, and the angle ofincidence of measurement light was set to 0 degrees. The transmittanceof light in the wavelength range of 380 to 780 nm in the axial directionperpendicular to the transmission axis of the multilayer laminated film(referred to as the “reflection axis”) was measured at each wavelengthat intervals of 5 nm. The reflection axis was determined according tocrossed Nicols search (650 nm). The average value of transmittance oflight in the wavelength range of 380 to 780 nm was determined, and thevalue obtained by subtracting the average transmittance from 100 wasdefined as the average reflectance in the direction of the reflectionaxis at normal incidence. When the average reflectance was 50% or more,the measured multilayer laminated film was determined to be capable ofreflecting light in the direction of the reflection axis. For use inoptics such as luminance-improving members, the average reflectance is85% or more, preferably 87% or more, and more preferably 90% or more.

(5) Maximum Transmittance Value at a Wavelength of 750 to 850 nm

For the obtained multilayer laminated film, the transmittance of lightin the direction of the transmission axis and the transmittance of lightin the axial direction perpendicular to the transmission axis(reflection axis) in a wavelength range of 300 nm to 1200 nm weremeasured to obtain optical spectra using a spectrophotometer (UV-3101PCand MPC-3100, manufactured by Shimadzu Corporation). The angle ofincidence of measurement light was set at 0 degrees.

The wavelength range of 750 to 850 nm overlaps with the visible lightregion (in particular, red light region) due to the shift of the spectrato the short-wavelength side when the film is viewed from an obliquedirection (direction at an incident angle of 45 to 60 degrees).Therefore, a large maximum value of transmittance of light in thiswavelength range represents a tendency of the occurrence of more notablecoloring of the multilayer laminated film when the multilayer laminatedfilm is viewed from an oblique direction. Notable coloring when themultilayer laminated film is viewed from an oblique direction means thatthe reflection wavelength range is narrow. Therefore, the maximum valueof transmittance of light in a wavelength of 750 to 850 nm is preferably53% or less, more preferably 50% or less, still more preferably 47% orless, and even more preferably 44% or less.

(6) Degree of Polarization

The visibility corrected polarization degree of the obtained multilayerlaminated film was measured using a polarizing-film measuring device(VAP7070S; manufactured by JASCO Corporation), and the obtained valuewas defined as a degree of polarization (P) (unit: %). For themeasurement, a spot diameter adjusting mask Φ 1.4 and anangle-adjustable stage were used, and the angle of incidence ofmeasurement light was set to 0 degrees, and calculation was made basedon the average transmittance of light (wavelength range: 400 to 800 nm)in the direction of the transmission axis and in the axial directionperpendicular to the transmission axis of the multilayer laminated film.The axis was determined according to crossed Nicols search (650 nm).

The degree of polarization (P) is preferably 77% or more. For use inoptics such as luminance-improving members, the degree of polarization(P) is preferably 78% or more, more preferably 80% or more, and stillmore preferably 85% or more.

Production Example 1: Polyester A

A polyester for birefringent layers was prepared as follows. Dimethyl2,6-naphthalenedicarboxylate, dimethyl terephthalate, and ethyleneglycol were subjected to a transesterification reaction in the presenceof titanium tetrabutoxide, and subsequently further subjected to apolycondensation reaction to prepare a copolyester in which 95 mol % ofthe acid component is a 2,6-naphthalenedicarboxylic acid component, 5mol % of the acid component was a terephthalic acid component, and theglycol component was an ethylene glycol component (intrinsic viscosity:0.64 dl/g; measured using o-chlorophenol at 35° C.; this also appliesbelow).

Production Example 2: Polyester B

A polyester for isotropic layers was prepared as follows. Dimethyl2,6-naphthalenedicarboxylate, dimethyl terephthalate, ethylene glycol,and trimethylene glycol were subjected to a transesterification reactionin the presence of titanium tetrabutoxide, and subsequently furthersubjected to a polycondensation reaction to prepare a copolyester inwhich 50 mol % of the acid component was a 2,6-naphthalenedicarboxylicacid component, 50 mol % of the acid component was a terephthalic acidcomponent, 85 mol % of the glycol component was an ethylene glycolcomponent, and 15 mol % of the glycol component is a trimethylene glycolcomponent (intrinsic viscosity: 0.63 dl/g).

Example 1

Polyester A for birefringent layers was dried at 170° C. for 5 hours.Thereafter, polyester B for isotropic layers was dried at 85° C. for 8hours. Thereafter, polyester A and polyester B were respectively fed tofirst and second extruders, and heated to 300° C. so that they were in amolten state. The polyester for birefringent layers was divided into 138layers, and the polyester for isotropic layers was divided into 137layers. A melt in a laminated state having 275 layers in total wasobtained using a multilayer feed block device equipped with comb teethfor alternately laminating the birefringent layer and the isotropiclayer and obtaining the layer thickness profile shown in Table 1. Whilethe laminated state was maintained, the same polyester as the polyesterfor isotropic layers was introduced to both sides of the melt from athird extruder toward a three-layer feed block to further laminate abuffer layer on both sides in the laminating direction of the melt (bothsurface layers of which were birefringent layers) in a laminated statehaving 275 layers. The feed rate of the third extruder was adjusted sothat the total of the buffer layers on both sides was 47% of the whole.The laminated state was further divided into two parts by using alayer-doubling block, and they were laminated at a ratio of 1:1, therebypreparing an unstretched multilayer laminated film having 553 layers intotal, including an intermediate layer inside the film and two outermostlayers on the outermost surfaces of the film.

The unstretched multilayer laminated film was stretched 5.9-fold in thewidth direction at a temperature of 130° C. The obtained uniaxiallystretched multilayer laminated film had a thickness of 75 μm.

Examples 2 to 8 and Comparative Examples 1 to 5

Uniaxially stretched multilayer laminated films were obtained in thesame manner as in Example 1, except that the multilayer feed blockdevice used was changed so that the layer thickness profiles shown inTable 1 were obtained.

In Comparative Example 1, the thickness was reduced in the region fromlayer number 29 to layer number 138 of the birefringent layer. Althoughthis region was not a monotonically increasing region, the ratio 1B/1Aof the slopes was calculated by regarding this range as a monotonicallyincreasing region 1B.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 BirefringentRefractive index nX — 1.849 1.849 1.849 1.849 1.849 1.849 1.849 1.849layer Thinnest layer Layer number — 1 1 1 1 1 1 1 1 (in monotonicallyLayer thickness nm 27 27 27 27 27 27 27 27 increasing region) Opticalthickness nm 50 50 50 50 50 50 50 50 Layer with an optical Layer number— 20 20 34 34 3 3 39 39 thickness of 100 nm Layer thickness nm 54 54 5454 54 54 54 54 (in monotonically Optical thickness nm 100 100 100 100100 100 100 100 increasing region) Thickest layer Layer number — 138 138138 138 138 138 138 138 (in monotonically Layer thickness nm 108 108 108108 108 108 108 108 increasing region) Optical thickness nm 200 200 200200 200 200 200 200 Average optical Thinner layer nm 75 75 75 75 75 7575 75 thickness Thicker layer nm 150 150 150 150 150 150 150 150 (inmonotonically increasing region) Slope of thickness 1A — 2.63 2.63 1.521.52 25.00 25.00 1.32 1.32 profile 1B — 0.85 0.85 0.96 0.96 0.74 0.741.01 1.01 (in monotonically 1B/1A — 0.32 0.32 0.63 0.63 0.03 0.03 0.770.77 increasing region) Isotropic Refractive index nX — 1.623 1.6231.623 1.623 1.623 1.623 1.623 1.623 layer Thinnest layer Layer number —1 1 1 1 1 1 1 1 (in monotonically Layer thickness nm 49 49 49 49 49 4949 49 increasing region) Optical thickness nm 80 80 80 80 80 80 80 80Layer with an optical Layer number — 119 100 119 100 122 80 122 80thickness of 200 nm Layer thickness nm 123 123 123 123 123 123 123 123(in monotonically Optical thickness nm 200 200 200 200 200 200 200 200increasing region) Thickest layer Layer number — 137 137 137 137 137 137137 137 (in monotonically Layer thickness nm 209 209 209 209 209 209 209209 increasing region) Optical thickness nm 339 339 339 339 339 339 339339 Average optical Thinner layer nm 140 140 140 140 140 140 140 140thickness Thicker layer nm 270 270 270 270 270 270 270 270 (inmonotonically increasing region) Slope of thickness 2A — 1.02 1.21 1.021.21 0.99 1.52 0.99 1.52 profile 2B — 7.78 3.78 7.78 3.78 9.33 2.46 9.332.46 (in monotonically 2B/2A — 7.63 3.12 7.63 3.12 9.42 1.62 9.42 1.62increasing region) Capable of reflecting light — Yes Yes Yes Yes Yes YesYes Yes Degree of polarization % 90.3 85.0 85.9 81.1 91.5 86.9 87.7 79.0Maximum transmittance value (750-850 nm) % 39.3 28.2 42.1 28.5 46.5 18.350.0 19.6 Average reflectance (380-780 nm) % 93.7 90.6 92.8 88.9 96.490.8 92.7 87.7 Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex.5 Birefringent Refractive index nX 1.849 1.849 1.849 1.849 1.849 layerThinnest layer Layer number 1 1 1 1 1 (in monotonically Layer thickness27 27 27 27 27 increasing region) Optical thickness 50 50 50 50 50 Layerwith an optical Layer number 29 20 39 39 44 thickness of 100 nm Layerthickness 54 54 54 54 54 (in monotonically Optical thickness 100 100 100100 100 increasing region) Thickest layer Layer number 29 138 138 138138 (in monotonically Layer thickness 54 108 108 108 108 increasingregion) Optical thickness 100 200 200 200 200 Average optical Thinnerlayer 75 75 75 75 75 thickness Thicker layer 84 150 150 150 150 (inmonotonically increasing region) Slope of thickness 1A 1.79 2.63 1.321.32 1.16 profile 1B −0.30 0.85 1.01 1.01 1.06 (in monotonically 1B/1A−0.17 0.32 0.77 0.77 0.91 increasing region) Isotropic Refractive indexnX 1.623 1.623 1.623 1.623 1.623 layer Thinnest layer Layer number 1 1 11 1 (in monotonically Layer thickness 49 49 49 49 49 increasing region)Optical thickness 80 80 80 80 80 Layer with an optical Layer number 115124 124 65 80 thickness of 200 nm Layer thickness 123 123 123 123 123(in monotonically Optical thickness 200 200 200 200 200 increasingregion) Thickest layer Layer number 137 137 137 137 137 (inmonotonically Layer thickness 209 209 209 209 209 increasing region)Optical thickness 339 339 339 339 339 Average optical Thinner layer 140140 140 140 140 thickness Thicker layer 270 270 270 270 270 (inmonotonically increasing region) Slope of thickness 2A 1.05 0.98 0.981.88 1.52 profile 2B 6.36 10.77 10.77 1.94 2.46 (in monotonically 2B/2A6.06 10.99 10.99 1.03 1.62 increasing region) Capable of reflectinglight No Yes Yes Yes Yes Degree of polarization 60.8 91.7 89.0 70.7 75.4Maximum transmittance value (750-850 nm) 86.4 55.9 56.9 17.5 21.8Average reflectance (380-780 nm) 69.5 94.3 92.6 85.3 86.3

As is clear from Table 1, the multilayer laminated films of the Exampleshad a high degree of polarization and a wide reflection wavelengthrange, compared to the multilayer laminated films of the ComparativeExamples.

The multilayer laminated film according to one embodiment of the presentdisclosure can achieve a high degree of polarization while maintaining awide reflection wavelength range by appropriately designing the opticalthickness of a birefringent layer and an isotropic layer that arealternately laminated. Accordingly, when the film is used as aluminance-improving member, a reflective polarizer, or the like, forwhich polarization performance is required, the film exhibits a highdegree of polarization over a wide reflection wavelength range.Therefore, more highly reliable luminance-improving members, polarizersfor liquid crystal displays, and the like can be provided.

The disclosure of Japan Patent Application No. 2018-182865, filed onSep. 27, 2018, is incorporated herein by reference in its entirety.

All of the documents, patent applications, and technical standardsreferred to in the present specification are incorporated herein byreference to the same extent in which these individual documents, patentapplications, and technical standards were specifically and individuallyindicated to be incorporated by reference.

1. A multilayer laminated film comprising a multilayer laminate in whicha birefringent layer comprising a first resin and an isotropic layercomprising a second resin are alternately laminated, the multilayerlaminated film being capable of reflecting light with a wavelength of380 to 780 nm due to optical interference caused from the laminationstructure of the birefringent layer and the isotropic layer, a series ofthe birefringent layers having a first monotonically increasing regionof optical thickness, wherein the first monotonically increasing regioncomprises a monotonically increasing region 1A in which the maximumoptical thickness is 100 nm or less, and a monotonically increasingregion 1B in which the minimum optical thickness is more than 100 nm,and a ratio 1B/1A of a slope 1B of the monotonically increasing region1B to a slope 1A of the monotonically increasing region 1A is more than0 and less than 0.8, a series of the isotropic layers having a secondmonotonically increasing region of optical thickness, wherein the secondmonotonically increasing region comprises a monotonically increasingregion 2A in which the maximum optical thickness is 200 nm or less and amonotonically increasing region 2B in which the minimum opticalthickness is more than 200 nm, and a ratio 2B/2A of a slope 2B of themonotonically increasing region 2B to a slope 2A of the monotonicallyincreasing region 2A is more than 1.5 and 10 or less.
 2. The multilayerlaminated film according to claim 1, wherein the monotonicallyincreasing region 1A has an average optical thickness of 65 nm or moreand 85 nm or less, and the monotonically increasing region 1B has anaverage optical thickness of 140 nm or more and 160 nm or less.
 3. Themultilayer laminated film according to claim 1, wherein themonotonically increasing region 2A has an average optical thickness of130 nm or more and 155 nm or less, and the monotonically increasingregion 2B has an average optical thickness of 250 nm or more and 290 nmor less.
 4. The multilayer laminated film according to claim 1, whereinthe ratio 2B/2A is more than 1.5 and less than
 5. 5. The multilayerlaminated film according to claim 1, wherein the ratio 2B/2A is 5 ormore and 10 or less.
 6. The multilayer laminated film according to claim1, wherein the multilayer laminated film has an average reflectance oflight polarized parallel to a reflection axis at normal incidence of 85%or more in a wavelength range of 380 nm to 780 nm.
 7. The multilayerlaminated film according to claim 2, wherein the monotonicallyincreasing region 2A has an average optical thickness of 130 nm or moreand 155 nm or less, and the monotonically increasing region 2B has anaverage optical thickness of 250 nm or more and 290 nm or less.
 8. Themultilayer laminated film according to claim 7, wherein the ratio 2B/2Ais more than 1.5 and less than
 5. 9. The multilayer laminated filmaccording to claim 8, wherein the multilayer laminated film has anaverage reflectance of light polarized parallel to a reflection axis atnormal incidence of 85% or more in a wavelength range of 380 nm to 780nm.
 10. The multilayer laminated film according to claim 7, wherein theratio 2B/2A is 5 or more and 10 or less.
 11. The multilayer laminatedfilm according to claim 10, wherein the multilayer laminated film has anaverage reflectance of light polarized parallel to a reflection axis atnormal incidence of 85% or more in a wavelength range of 380 nm to 780nm.